Geology of the seismogenic subduction thrust interface

Abstract

A microseismically active layer of underthrust sediments is commonly inferred along subduction thrust interfaces. The exhumed Chrystalls Beach Complex, in the Otago Schist, New Zealand, may be analogous to an actively deforming underthrust rock assemblage. The complex contains asymmetric competent lenses of sandstone, chert and basalt enclosed in a cleaved mudstone matrix. Continuous fabrics such as folds, boudins and asymmetric phacoids formed by distributed cataclasis and dissolution-precipitation creep. Discontinuous deformation is evident in an extensive fault-fracture mesh involving mutually cross-cutting subvertical extension veins and subhorizontal slickenfibre shear surfaces.The Hikurangi margin provides an example of along-strike variations in seismic style, possibly related to heterogeneous fluid-pressure state and interface geology. In both the ancient and active subduction-related shear zone, fluid-pressure state appears to be a critical control on frictional failure, which primarily occurs on weak, fluid-overpressured discontinuities. Continuous, aseismic deformation occurs where other mineral deformation mechanisms, such as dissolutionprecipitation creep, are preferred. The geometry and composition of the underthrust rock assemblage appear to be first-order controls on megathrust fluid-pressure distribution, bulk rheology and dominant deformation mechanism, and thus may be significant controls on megathrust seismic style.

Cascadia subducting plate fluids channelled to fore‐arc mantle corner: ETS and silica deposition

“…This 5–10 km thick distribution also is consistent with inferences from dipping reflectors in multichannel seismic reflection data above the subducting oceanic crust of a thick shear zone in the ductile regime where tremor commonly occurs [e.g., Nedimović et al ., ]. Thick shear zones also are often found in paleosubduction thrusts [e.g., Fagereng , ; Cloos and Shreve , ]. The depth to the top of the subducting oceanic plate has been debated for some time.…”

Early exhumation of underthrust units near the toe of an ancient erosive subduction zone: A case study from the Northern Apennines of Italy

“…The authors also thank Filippo Panini for fruitful discussions. This research was fi nancially supported by Ministero dell'Istruzione, dell'Università e della Ricerca (MIUR) research grants PRIN 2008 and PRIN 2010/2011 …”

“…Thus, the plate boundary can cut through material characterized by different compositions, lithification/metamorphic conditions, and fl uid pressure. Some types of changes can directly affect fault rheology and shear strength, which are key control factors on earthquake nucleation and rupture propagation (Fagereng, 2011).…”

Geology of the earthquake source: an introduction

“…2a) (Sibson 1986;Pavlis et al 1993), but it cannot be said with certainty that the slip leading to dilatancy happened at seismic rates (Cowan 1999). Other structures that may be associated with seismic slip include rounded spherical aggregates in clayey gouge, observed in laboratory shear experiments at fast slip rates (Boutareaud et al 2008;Ujiie et al 2011) and in samples from natural, seismogenic faults (Boullier et al 2009;Boullier 2011), and incrementally developed hydrothermal veins (Boullier & Robert 1992;Fagereng 2011). Unfortunately, these structures are either difficult to recognize in exhumed rocks, which have experienced overprinting by further deformation at depths shallower or deeper than the seismogenic regime, or may also be interpreted as resulting from slower slip rates.…”